In the design of MOS amplifiers it is important in the initial design that the amplifiers be kept small and dissipate as little power as possible. This is particularly true in transconductance amplifiers, which are repeated a large number of times on a semiconductor chip. These amplifiers have already been reduced in both die size and power by the exclusion of the output stage, and may be further improved by a judicious choice of the compensation technique.
It is also important in each amplifier design that the unity gain bandwidth not greatly exceed the required settling time (as dictated by the clock frequency). Excess amplifier bandwidth results in more broad-band noise being passed through each stage and, subsequently, being aliased into the passband. In amplifier types which exhibit a poor settling response or which are sensitive to process and/or ambient conditions, a large bandwidth may actually be required in order to guarantee that minimum settling requirements are met in the presence of varying operating conditions and process parameters. Two of the parameters that are subject to change over processing are K' and V.sub.TO. Both of these parameters are functions of substrate doping levels and the capacitance of the oxidation layer. As these terms vary, the transconductance of the active devices on a MOS chip also vary and, depending upon the configuration of an amplifier, the poles and zeros vary, thus changing the frequency response of the amplifier.
In a paper by W. C. Black, D. J. Allstot and R. A. Reed entitled "High Performance Low Power CMOS Channel Filter" in the IEEE Journal of Solid State Circuits, Volume SC-15, No. 6, Dec. 1980, a compensation technique is disclosed for adjusting the position of the low frequency right half plane zero. This is a common problem in MOS amplifiers due to the lower transconductance MOS devices. The zero is positioned by the inclusion of a resistor in series with the compensation capacitor. This paper outlines a technique that utilizes an active feedback transistor which is controlled by an external bias circuit. This provides a tracking compensation scheme that is independent of process variations.
The active device utilized as the feedback resistor is controlled by making its size, and the size of an additional transistor in the amplifier, a certain multiple of the sizes of two transistors in the bias circuit.
In view of the above problem there exists a need for a tracking scheme that is independent of processing and places no size requirements on the amplifier transistors in order that they can be optimized to meet other requirements.